The present invention relates to a coaxial transmission line or cable radiating as well as transmitting electromagnetic high frequency energy. Such a cable is comprised of an inner conductor, an outer conductor disposed in concentric relation to the inner conductor and a dielectric spacer between the two conductors, the outer conductor being constructed so that the cable transmits as well as radiates h.f. energy, thus acting as a regular transmission line as well as an antenna.
The concentric conductor system of a coaxial cable will or will not radiate high frequency energy, depending essentially on the configuration of the outer conductor. Of course, all transmission lines have radiation losses to some extent, which are minimized as much as possble for regular coaxial cable. In some instances, however, it is desirable to have the cable acting as an antenna radiating a particular portion of the transmitted power over its entire or a well defined extension. Cables radiating high frequency are used to transmit radio frequencies to a mobile receiver. For example, such cables are used inside railway tunnels to insure the transmission of signals to a train while passing through the tunnel. It is, of course, required that such a cable radiates as uniformly as possible over its extension while that portion of energy which is not being transmitted by such radiation be transmitted through the cable at a low loss.
Radiating cable of this type are, for example, installed along railway tracks where the cable is particularly mounted to the ties or on poles placed alongside the track. In the case of a tunnel, the cable may be affixed to and along the wall of the tunnel. The conventional radiating cables are, for example, constructed as unshielded symmetrical conductors. However, such a radiating cable is directly exposed to the environment and to weather conditions so that the losses, as well as the transmission characteristics, vary greatly. This is particularly true if such a cable is affixed directly to the ground.
The German printed patent appliction No. 1,044,199 discloses a high fequency radiating cable which is basically of coaxial configuration, but the outer conductor is provided with a longitudinal gap or slot which runs parallel to the axis of the cable. The outer conductor, therefore, is an incomplete tube enveloping only partially the insulation surrounding the inner conductors. High-frequency energy is radiated through the slot or gap.
The German printed patent application No. 2,022,990 discloses a high-frequency cable in which the outer conductor is constructed by winding a ribbon or a wire-like material around the dielectric spacer on the inner conductor. High frequency energy radiates through the resulting gaps or openings. Either of these radiating cables exhibits the drawback that in the case of continuous transmission standing waves are set up along the outer conductor, thus, producing strong variations in the strength of the outer electric field in longitudinal direction of the cable. Consequently, the radiated signals are received under conditions of significant inerference.
The printed German patent application No. 1,690,138 discloses a cable constructed for discontinuous operation. This cable has again a tubular outer conductor which is provided with a slot from which emerges an electromagnetic field. The slot extends obliquely to the axis of the cable but its direction changes, resulting in a zig-zag line. Metal rings short-circuit the slot wherever it changes direction. The purpose of this particular configuration of the slot is to suppress the axial component of the electromagnetic field so that it will not be radiated and, ultimately, an attempt is made here to establish a more uniform field. It was found, however, that in practice this hoped-for effect does not occur or it occurs only to a very significant extent. Moreover, such a cable is very difficult to manufacture because different kinds of cable, particularly different dimensions thereof, require different configurations of the respective slot. Hence, the stamping tools making this slot will differ. Also, the short-circuiting metal rings must be individually placed on the cable; such a procedure precludes a continuous production.